The present invention is directed to a system and method for automatically detecting the dimensions or edge of a document being scanned by a scanning system. More particularly this invention is directed to an overlapping sensor array which provides positioning accuracy independent of tracking distance and eliminates dead zones in large area scanning.
Traditionally, the term copier, when used in the office equipment context, refers to a light lens xerographic copier in which paper originals are reproduced. Images of the original document are focused on an area of a photoreceptor, which is subsequently developed with toner. The developed image on the photoreceptor is then transferred to a copy sheet which in turn is used to create a permanent copy of the original.
Digital copiers are now available that perform the same functions as a light lens copier, except that the original image to be copied is not directly focused on a photoreceptor. Instead, with a digital copier, the original image is scanned by a device generally known as a raster input scanner (RIS) which is typically in the form of the linear array of small photosensors.
The original image is focused on the photosensors in the RIS. The RIS converts the various light and dark areas of the original image to a set of digital signals. These digital signals are temporarily retained in a memory and then eventually used to operate a digital printing apparatus or for other reproduction purposes. The digital printing apparatus can be any known type of printing system responsive to digital data.
With the migration of the copying and scanning systems to a digital base system, the systems face different problems than from the light lens or analog copying systems. More specifically, in a digital scanning system, the scanning system needs to locate the document in the operational sequence of the machine. This needs to be accomplished accurately for image processing and other functions.
In the past this tracking function was accomplished by an array of sensors which generate pixel signals representing the entire page. This creates a significant amount of extraneous data when all that is needed are data indicative of the edge of the document. A similar approach has been used utilizing individual or groups of position sensors place in a location in the document path at which the document is made to cast a shadow over the sensor which alters the output of the sensor and thereby generates an indication of the location of an edge of the document.
Position sensors of this type are available from, for example, SDT Sensors, Inc. of Hawthorne, Calif. These lateral photodiode position sensitive detectors comprise a semiconductor strip of silicon having a composition and structure that, when stimulated by the impingment of light, collects a current at each end of the strip. By a simple mathematical comparison of the relative currents at each electrode, the centroid of the light impinging on the strip can be determined. This data allows the position of an edge of a document to be determined as the object alters the distribution of light on the sensor strip. A problem with such sensors is that their accuracy falls off as the centroid of light approaches either of the electrodes. This hinders the assembly by abutment of such sensors into arrays as dead spots will occur that detract from the reliability and accuracy of the generated data.
It is an object of this invention to provide a simple sensing device for tracking the position of an edge of a document or other object as it proceeds through the operational cycle of a machine such as a printer or multifunction digital scanner. It is a further object of this invention to construct an array of semiconductor position sensors to accomplish edge tracking without dead spots. The invention may be used to determine the position of any object along an object transport path with a prescribed resolution throughout. It is particularly suited for large area tracking.
Semiconductor position sensors consisting of strips of light sensitive silicon composition film are constructed with electrodes at either end of each strip that collect currents generated by impinging light on the sensors. The sensor strips are strategically positioned in the path of an object whose position is to be monitored. As the object passes by the sensor the impingement of light on the sensor is altered. The currents generated are indicative of the position of the centroid of light impinging on the strip. From this data, the position of the edge of an object passing the position sensor can be monitored. According to this invention, multiple position sensors are assembled into an array in which the electrodes at the ends of individual sensors are positioned in overlapping interleaved relation to adjacent sensors. The array is assembled to form a sensing plane in which each sensor is located next to another sensor, but offset so that an edge image or shadow, passing from one electrode to the other of a particular sensor, is always positioned on the central zone of high accuracy of at least one sensor. This can be accomplished by physical placement of individual sensors or by shaping the sensor strips to allow the necessary interleaving. It also can be accomplished by a patterned installation of the electrodes to form sensing zones on a strip in which each zone can function substantially the equivalent of individual sensors.